Matthew 27:5

And he cast down the pieces of silver into the sanctuary, and departed; and he went away and hanged himself. And the chief priests took the pieces of silver, and said, It is not lawful to put them into the treasury, since it is the price of blood.

The casting of the silver into the sanctuary fulfilled Zechariah's prophecy (Zechariah 11:12). See under Matthew 27:10, below. Other things forbidden to the treasury were the hire of a harlot and the price of a dog (Deuteronomy 23:18). The capacity of those men to "strain out the gnat and swallow" (see under Matthew 23:23ff) is almost unbelievable. They were not above hiring perjured witnesses, bribery, plotting to murder the Son of God, or doing any other evil thing that might have seemed expedient; but to take back their own money from repentant Judas, THAT was unlawful! Furthermore, the restriction against blood-money being placed in the treasury does not appear to be one of God's restrictions, but one of their own! Deuteronomy 23:18, usually cited in this context, says nothing of the "price of blood." That was probably just another instance of their having made their own traditions of more importance than God's word.